Excellent! Thanks for the answers. When I've explored a bit more I might implement the overview sizes just so we can match downstream tools (the current motivation is like-for-like performance comparison, I haven't looked there but I think odc goes very low level to eke out speed).
Cheers, Mike On Wed, Mar 20, 2024 at 10:39 AM Even Rouault <even.roua...@spatialys.com> wrote: > Michael, > > Hi, can we specify overview sizes exactly? > > No, the BuildOverviews() interface onlys take an array of overview > factors. It is up to the driver implementation to decide how it computes > the overview size from the main raster size and overview factor. > > The COG driver is a bit of a special case, since it automagically compute > overviews if they don't exist in the source dataset, and doesn't take an > explicit list of overview factors. It simply divides by two successively, > with floor truncation: > https://github.com/OSGeo/gdal/blob/master/frmts/gtiff/cogdriver.cpp#L1092 > > I have this odd grid that is 36000x17999, and I get consequently yucky > overview sizes: > > gdal_create -outsize 36000 17999 -ot Int8 -co SPARSE_OK=YES -a_srs > EPSG:4326 -a_ullr 0 17999 0 36000 weird.tif > gdal_translate weird.tif cog.tif -of COG > gdalinfo cog.tif | grep Overviews > # Overviews: 18000x8999, 9000x4499, 4500x2249, 2250x1124, 1125x562, > 562x281, 281x14 > > Should I worry about this? > > Probably not ? :-) > > Can the sizes be specified via Translate? (I can see how they could be > generated externally and bundled, but I'm looking for a way at this high > level). > > I guess someone could potentially add a OVERVIEW_SIZES=W1xH1,W2xH2,... > creation option to the COG driver if that was really needed... > > > ( Maybe I shouldn't even care about the sizes ... and maybe I should > resample the grid to be a better overall size, but I consider that out of > scope here.) > > I'm comparing with other COG creators, e.g. odc which makes tidy overviews > ( I will explore how it's getting done). > > It looks like they might use (previous_size + 1) / 2 (or ceil rounding) > > -- http://www.spatialys.com > My software is free, but my time generally not. > > -- Michael Sumner Software and Database Engineer Australian Antarctic Division Hobart, Australia e-mail: mdsum...@gmail.com
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